1. Field of the Invention
The present invention relates to a process for passivating polysilicon, and more particularly to a process for passivating polysilicon by means of high pressure annealing using a fluorine-containing gas, a chlorine-containing gas, an oxygen-containing gas, a nitrogen-containing gas, or mixtures thereof for fabrication of polysilicon thin film transistors.
2. Description of the Prior Art
In polysilicon thin film transistors (p-Si TFTs), the grain boundary of polysilicon or the defect density in the grains plays an important role in the electric properties of TFTs. Reducing the defect density can greatly enhance the electric properties of the TFTs. One method of reducing the defect density is to increase the polysilicon grain size and crystallinity. Another method is to introduce other atomic components into polysilicon to fill in the defects, this process is generally referred to as passivation.
Due to small volume and high diffusion rate, hydrogen atoms have been used frequently to passivate polysilicon to fill the defective atoms. Such passivation is referred to as hydrogenation. Hydrogen atoms, however, diffuse out at high temperature, rendering the previous hydrogenation ineffective. Therefore, after hydrogenation, the device generally cannot be subjected to a process with higher temperature (such as 450° C.). Therefore, hydrogenation is generally performed when the device is almost completed. However, at this time, there are several layers of coating on the polysilicon thin film channel, which greatly reduces the hydrogenation efficiency. In addition, the bonding between hydrogen and silicon atoms is very weak, and easily broken during device operation. Therefore, using hydrogen atoms to fill in defects frequently reduces device reliability.